Gene Flow

Natural Populations Are Not Single Well-Mixed Gene Pools

A classic example of polymorphism is the banding patterns in the snail Cepaea nemoralis, one of the few such examples before molecular markers became available. Jones et al. (1977) review the multiple explanations for spatial variation in banding pattern; for a more recent account, incorporating molecular data, see Davison and Clarke (2000).

The example of Figure 16.1B,C is from Ochman et al. (1983). Shell polymorphism is correlated both with temperature (as in Fig. 16.1B) and with camouflage: in some areas at least, shells tend to match their background. However, shell color and banding also sometimes show “area effects,” such as those illustrated for allozyme frequencies in Figure 16.1C, in which large patches show distinct combinations of allele frequencies.

The Rate of Diffusion of Genes Is Measured by σ2

In two dimensions, the distance moved by a gene is measured along some particular axis (x, say). The rate of gene flow in that direction is measured by the variance of that component of distance ( = E[x2], say). The variance of the total distance moved is the sum of the variances in the two directions. This is because, by the Pythagorean theorem, the total distance moved, d, is given by d2 = x2 + y2. Thus, the variance of the distance moved is the sum of contributions from movements along the x and y axes (E[d2] = E[x2] + E[y2] = + ). In this chapter, we use σ2 to denote the variance along the axis of interest (, say). For example, this might be the distance measured in the direction of a cline.

Dobzhansky and Wright (1943, 1947) made some of the first large-scale measurements of gene flow and were the first to relate this measurement to patterns of genetic variation—in their work, to variation in the frequency of recessive lethals. The history of their collaboration is described by Lewontin et al. (1981), together with reprints of their classic papers.

Coyne et al. (1982) give a nice example of long-distance movements of Drosophila to isolated oases in the Mojave desert.

Selection can be detected by comparing FST across genes: Higher FST may be caused by diversifying selection. A good example comes from the periwinkle, Littorina saxatalis (Using FST to Detect Selection in the Periwinkle, Littorina saxatalis). The same approach has been used in comparisons across the human genome to indicate genes that may be selected differently on different continents (see Fig. 26.11).

Detecting selection by comparing the extent of between- and within-population divergence, as measured by FST, is similar to the McDonald–Kreitman (Box 19.1) test, which compares divergence within and between species.

Rates of Gene Flow Can Be Estimated from FST

In animals, FST will be lower for mitochondrial genes than for nuclear genes if females move less than males, simply because mitochondria are inherited only through females. The converse applies to the Y chromosome, which is passed down only through males. As well as these differences in gene flow (m), the effective population sizes also differ between genes that are inherited in different ways.

Genealogies in Structured Populations

Genealogies Are Distorted by Population Structure

Charlesworth et al. (2003) review the effects of population subdivision on gene genealogies. This theory carries over from the “classical” results of Wright, Malécot, and others, which are framed in terms of identity by descent or variance in allele frequency.

Slatkin M. and Barton N.H. 1990. A comparison of three methods for estimating average levels of gene flow. Evolution43: 1349–1368.

Szymura J.M. and Barton N.H. 1991. The genetic structure of the hybrid zone between the fire-bellied toads Bombina bombina and B. variegata: Comparisons between transects and between loci. Evolution45: 237–261.

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